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A theoretical model is presented to simulate the general behavior of the metal ignition process. The model includes many physical mechanisms (such as the various heat transfer rates, reaction rate, and the effect of oxide formation on the reacting surface) that are known to have influence on metal ignition. Based on numerical solutions, three separate types of metal ignition are identified. They are (1) external heating controlled ignition, (2) convection controlled ignition, and (3) convection and transition temperature controlled ignition. For certain types of metal ignition, such as the external heating controlled ignition, the ignition temperature appears to be an ineffective parameter in illustrating the relative ignitability of the different materials. For such cases, the concept of a minimum ignition heat flux is shown to be useful in correlating the relative ignitability.
Test data for five different alloys generated from the friction-rubbing test apparatus at the White Sands Test Facility are analyzed based on the present model. Qualitatively, almost all observed ignitions are shown to be external heating controlled. Tests with lower rotational speed and higher oxygen pressure are needed to determine the minimum ignition heat flux for the five alloys. Based on the current set of data, some conclusions concerning the minimum ignition heat flux of Type 316 SS and a preliminary ranking of the five alloys in terms of their relative ignitability are generated. These conclusions, however, require further verification by additional test data.
metals, heat transfer, ignition temperature, ignition, oxide layer, minimum ignition heat flux, metal ranking
Associate professor, University of California, Santa Barbara, CA